1. Field of the Invention
The present invention relates to a band pass filter, and more particularly to a band pass filter for use in communication devices.
2. Description of the Related Art
A band pass filter is a component which is needed to prevent interference of signals and effectively utilize a frequency. In the field of communications, performance of a filter is particularly important, as it determines an effective use of a frequency which is an important resource. That is, in regard to an electromagnetic wave transmitted/received by an antenna, an out-of-band signal is cut by a reception filter or a transmission filter, thereby greatly reducing interferences with an adjacent signal. In order to most effectively cut the out-of-band signal, a filter which can clearly separate each signal is desirable. However, in a high frequency band in particular, a super sharp cut filter is desirable in order to cut an adjacent signal in a very narrow band, but realization of such a very narrow band super sharp cut filter is very difficult.
Usually, a band pass filter on an RF stage is constituted by using many resonators. In the band pass filter constituted by many resonators, types of filter characteristics to be realized are determined by a value given to each coupling between the resonators. Further, whether the resonators are correctly coupled with each other determines whether the designed characteristic can be realized. In particular, in a narrow band filter that coupling between the resonators is very weak, coupling between the resonators is important.
There has been conventionally known a filter using a planar structure circuit as typified by a microstrip line, a strip line and others. For example, IEEE Microwave Theory and Techniques Symposium Digest (1998), p. 379 discloses a Chebychev filter that the number of path which couples the resonators is determined as one. In such a filter, realization of a narrow band is achieved by spatially increasing a distance between the resonators. Furthermore, IEEE Transactions on Microwave Theory and Techniques, Vol. 44 (1996), p. 2099 discloses a pseudo-elliptic function type which can suppress an insertion loss and constitute a sharp cut filter. This type of filter can be realized by introducing non-adjacent coupling to a filter such as a Chebychev filter having one path of signals and bringing in a shortcut path. Moreover, there has been developed a filter which adopts not only simple spatial coupling as strong non-adjacent coupling between resonators but carries out coupling through a transmission line path coupled with a resonator by using a short-length section such as disclosed in IEEE Microwave Theory and Techniques Symposium Digest (2000), p. 661, and a sharp cut type high-quality filter with a relatively broad band is realized. However, achieving both the very narrow band and the super sharp cut is difficult.
As described above, realization of a very narrow band super sharp cut filter is very difficult, by using a conventional filter. The reason will be described hereinafter as problems in the prior art.
There are two problems when realizing the super sharp cut filter. For example, in a Chebychev filter or the like which adopts a structure that coupling between resonators based on a gap is used and the number of path of couplings is one, such as disclosed in IEEE Microwave Theory and Techniques Symposium Digest (1998) p. 379, all the couplings become weak when each distance between the resonators is increased, but coupling of the resonators other than adjacent resonators does not become sufficiently weak. Therefore, the characteristic is disadvantageously disrupted when the coupling is adjusted by using the distance between the resonators to obtain a very narrow bandwidth filter. Additionally, since the distance between the resonators must be largely increased, the filter itself becomes large in size, a problem of a limitation in size of a substrate and the like restricts the design. Also, the sufficient number of resonators cannot be assured, and hence the sharp cut cannot be realized.
Another important problem becomes apparent when configuring the very narrow band sharp cut filter with a low insertion loss. In the regular Chebychev type filter, the number of resonators is increased in order to realize the sharp cut, but this is very disadvantageous in terms of the loss in case of the narrow band, and the insertion loss is greatly increased.
In order to reduce the insertion loss, it is necessary to constitute such a pseudo-elliptic function type which can suppress the insertion loss and configure the sharp cut filter as disclosed in IEEE Transactions on Microwave Theory and Techniques, Vol. 44(1996), p. 2099. This type of filter can be realized by introducing non-adjacent coupling to a filter, such as a Chebychev filter, having one path of signals and bringing in a shortcut path. Therefore, when a narrow band filter is tried to be realized, since weak non-adjacent coupling is introduced to the resonators which are originally connected by weak coupling, parasitic coupling is also generated to resonators other that those which should be coupled. This considerably disrupts the characteristic, and there occurs a problem that the sharp cut pseudo-elliptic function type filter cannot be successfully realized in the narrow band.
On the other hand, there has been developed such a filter which performs not only spatial coupling as strong non-adjacent coupling between the resonators, but also coupling through a transmission line path connected with the resonators via short-length sections, as disclosed in IEEE Microwave Theory and Techniques Symposium Digest (2000), p. 661. With this filter, a relatively-broad band sharp-cut high-quality filter can be realized. In this filter, however, spatial coupling between the resonators is also used for coupling between the adjacent resonators, but all the designed weak couplings are hard to be taken, thereby making it difficult to realize the very narrow band filter successfully. Additionally, in regard to non-adjacent coupling based on this transmission line path, there is a serious problem. This is a problem that an original resonance frequency of the resonators deviates by adding a transmission line path for coupling. In the very narrow band filter, since the band is originally very narrow, the filter is very sensitive to spatial distribution or the like of material parameters, adding such a deviation of the resonance frequency to this property results in a serious problem. For example, in the case of coupling the resonators, when a center frequency of each resonator is out of this band, which is assumed to be very narrow, realization of the band pass filter becomes very difficult.
As described above, the very narrow band sharp cut filter using a planar structure circuit is hard to realize based on only the prior art.